Rear-projection screens have to a great extent been used for video-projection devices, micro film readers, data equipment and flight simulators.
From the description of U.S. Pat. No. 3,279,314 and U.S. Pat. No. 2,738,706, rear light projection screens with lens tops are recognized by the front side, i.e. The side facing the observer. Common for both patents are that the individual lens tops on their inside are totalreflecting towards the parallel light of the projectors, ad the light--after total reflection--streams out of the lens tops. By observing a cross section through the lens tops it will be seen that the individual lenses form mutual V-shaped grooves. It is known from the above patents to fill these grooves with black colour or some other media containing light impenetrable material in order to increase the contrast.
The most essential difference from the above-mentioned patents and the present invention is that in the above-mentioned patents, the rear sides of the screen elements, i.e. those sides facing the projector are fully plane and thus do not contain any lens structure for deflecting the light. Another essential difference is that the above-mentioned U.S. Patents have cone-shaped lens tops, while the present invention has on the rear side of the foremost element horizontal outwardly extending, elongated convex lenses having its focal point congruent with the foremost elements front plane, where the light is emitted from the tops of the projections.
Further it is known from U.S. Pat. No. 2,618,198 to have a screen consisting of two screen elements where each screen element has groups of parallel cylindrical lenses facing the projectors, and further that each screen element's cylindrical lenses are placed at right angles to each other. The element, closest to the projectors, has on its rear side vertical outwardly extending, elongated convex lenses, and the focal point of which is congruent with the front plane of the screen foremost element. In order to obtain a good contrast effect, the screen front is applied with black covered strips as wide as possible. The space between the covered strips is for the purpose of emitting the light for the observer to see and which forms the picture concerned. The front side element is completely identical to the rear side element. However, the convex outwardly extending lenses are in a horizontal plane when the screen is applied in its user position.
Despite the fact that a so called Fresnel lens is applied in connection with the screen of the above-mentioned U.S. patent for paralleling of the light from the projector, this screen construction will only be applicable when "one" projector is used, in that the light beams from more than one projector will not all be parallel to one another and normal to the surface of the Fresnel lens, but will leave the Fresnel lens at an angle corresponding to the angle that the individual projectors' optical axes form mutually.
This again will have the consequence that light beams, which do not penetrate perpendicularly into the rear side of the picture forming foremost element, after deflection by the convex lenses on the reverse side of the foremost element will be displaced at an angle corresponding to the angle between the concerned projector's optical axis and the normal to the rear side of the foremost element and thus disappear in the black covered stripes.
The present invention further differs in that the surface for the emitting light does not comprise a lens structure.
It is a purpose of the present invention to prevent room light from penetrating into the screen and to reduce interial reflections in the screen's foremost element to increase the contrast.
It is known to produce a projected TV-picture by turning three projectors each of a separate color (red, green, blue) towards a transparent projection screen. The three projectors are usually positioned next to each other horizontally, and the forwarded, enlarged picture is turned towards the transparent screen. Because the three projectors are placed next to each other, their optical axes form angles with each other. Normally the optical axes form angles with each other from 7.degree. to 12.degree. depending on the size of the picture tube and the distance from picture tube to transmission screen.
Most designers of projection-TV place the green picture tube between the red and the blue so that the optical axis of the green picture is projected perpendicular to the transmitting screen. This has the effect that the optical axes of the blue and the red picture tube deviate e.g. by 9.degree. in comparison with the optical axis of the green picture tube.
It is known to construct a TV-rear light projection screen as shown on FIG. 3, with the parallel convex lenses 18 and 19 vertically running, and where 15 indicates a Fresnel lens for paralleling the light, 16 indicates the picture forming screen with the lens 18, the focal point of which is at lens 19's surface and where, between the convex lenses 19 and 19a, there are placed projections 20 for applying the covered stripes to increase the contrast.
As lens 18 only serves the purpose of spreading the light horizontal, the screen of the well-known construction shown on FIGS. 3, 4, 5 and 6 is applied with powdered glass or SiO.sub.2 for spreading the light in vertical direction.
These refractive particles are often evenly spread over the entire thickness of the screen element with the effect that not all rays will strike lens 19 at a well defined focal point, see FIG. 4, but will be deflected in undesired directions, and therefore, will have a bad effect on the picture quality. The light spreading particles are not marked on FIG. 3, 5 and 6, but are illustrated on FIG. 4 by the light beams 22, 23 and 25 winding through the screen, and further, the spreading of the light is illustrated by the vector crowd 24 and 24a. In addition it can be seen that the light beam 23, originating from the red or the blue light source is lost due to inner reflection, because the angle of incidence .alpha. is 7-10.degree. from the optical axis of the green (center) light source.
FIGS. 5 and 6 show the path of rays penetrating into the screen and being thrown back to the observer. For clearness purposes, the Fresnel lens 26 in FIG. 5 has the concentric grooves towards the projectors. It can be seen that the light beam 27 is total totally reflected on the Fresnel lens 26's surface 26a. It is known among professionals to dull the front side 28, see FIG. 6. This dulling shall prevent inconvenient reflection from the surroundings. The irregular surface, spreading the light, will reduce the contrast considerably which is illustrated by the vector crowds 29a, 30 and 31a, 32. The explanation of this contrast reduction is that an irregular surface will always have an angle in relation to the screen surface, so that no matter where in the room the light beam comes from, it will penetrate the surface 28.
It is known among professionals that the picture forming ability of a rear light projection screen is reduced when it is used in a very illuminated room. It is further known that spreading of the light in the screen, of the light that is projected upon the screen, is spread outside the desired illuminated area, in that the light can move from the illuminated groove to the neighbouring grooves, see FIGS. 4, 5 and 6. The same applies to light falling into the screen from the front side of the screen of which some of the light is lost inside the screen, while some is spread to the neighbouring grooves or is reflected from the Fresnel lense as shown in FIGS. 5 and 6, and after that is reflected out to the observer.
Both situations have inconvenient effects on the picture formation, hereunder especially fading of the shades.
It is the purpose with the present invention to prevent the mentioned inner reflection and thereby increase the contrast.
According to the invention, this contrast improvement is obtained by providing the rear side of the foremost screen element with linear horizontally placed upright convex lenses, the purpose of which is to gather the arriving light beams and direct them towards the space of the horizontal forerunning cams on the front side, and on whose tops the light comes out and between which there are V-shaped grooves and whose sides are applied with a medium impenetrable to light.
The rear light projection screen, according to the present invention, is characteristic by being a transmitting lens screen without inner reflecting surfaces in the screen, and where each individual lens element on the picture forming or front side of the foremost screen element cannot reflect light to the neighbouring lens, because the individual picture forming lenses are separated by V-shaped grooves.